Fuel tank systems for aircraft commonly comprise one or more fuel tanks and a vent tank. The vent tank may also be referred to as a surge tank. The vent tank is arranged to provide venting means to vent the fuel tanks to atmosphere. Vent tanks are arranged to allow air to flow from atmosphere into the fuel tanks as they are drained and to allow air, fuel vapour or inerting gasses or a mixture of these to flow out of the tanks to atmosphere as the tanks are filled. In addition, during such fill or drain operations, vent tanks are also commonly required to equalize pressures in normal operation due to ambient pressure changes.
Vent tanks are commonly arranged with a predetermined liquid fuel storage capacity. This capacity provides storage for liquid fuel forced from the fuel tanks and vent system into the vent tank, for example, as a result of an aircraft manoeuvre or overfilling of the fuel tanks during refueling of the aircraft. While vent tanks are commonly arranged with means for returning such surges of fuel to the fuel tanks in a controlled manner, if the liquid fuel capacity of the vent tank is exceeded the fuel is vented to atmosphere.
One problem with such surges of liquid fuel is that they may be at a relatively high pressure during filling. If such pressure is not properly vented then the fuel tank system may be physically damaged. This is a particular problem if the fuel tank system comprises an integral part of the aircraft structure. The problem may be exacerbated when fuel is cold and thus more viscous and therefore more resistant to flow through the venting system.
Another problem exists in that fuel storage space in an aircraft is limited and valuable. Vent tanks may be located in areas that are not suitable for fuel storage and generally need to be as space efficient as possible while providing the required vent capacity. For example, vent tanks are commonly located in the wing tips of an aircraft where space, particularly depth, is restricted.